1. Field of the Invention
The present invention relates to a molded component for protecting a radar apparatus, especially to a molded component for the beam path of a radar apparatus disposed behind the front grill of an automobile.
2. Background Art
In general, antennas that transmit and receive radio waves in communication devices, radar devices, and the like have rarely been restricted in terms of the design of their bodies and peripheral structures, since their functions have been given priority. For example, a rod antenna whose shape is exposed is used for automobile radio antennas and the like. There may be a case where the antenna is hopefully hidden depending on the location in which the antenna is attached. For example, in a radar system or the like that measures obstacles in front of an automobile and the distance between automobiles, the antenna is preferably disposed in a central position at the front of the automobile in order to make the most of its performance. In this case, although the antenna is likely to be installed near the front grill of automobile, it is preferable that it not be possible to view the antenna from the outside in terms of design.
An auto cruise system involves technology for controlling the distance between vehicles by accelerating or decelerating one's own vehicle, in which a sensor mounted in the front of the vehicle measures the distance and a relative speed between a vehicle in front and one's own vehicle, and the throttle and brakes are controlled based on this information. The auto cruise system has recently attracted attention as one of the core technologies of the intelligent transportation system (ITS) for easing traffic congestion and decreasing traffic accidents. In general, a radio transmitter-receiver device such as a millimeter-wave radar is used as a sensor for the auto cruise system.
As shown in FIG. 7, a radar apparatus 100 mounted in an automobile is usually disposed behind a front grill 101. An emblem 102 of an automobile manufacturer or a specific decoration is disposed on the front grill 101. Millimeter waves from the radar apparatus are radiated forward via the front grill and the emblem, and reflections from an object return to the radar apparatus via the front grill and the emblem.
Therefore, it is preferable to use materials and paints that provide low radio transmission loss and a predetermined appearance for the front grill and the emblem, especially where the beam path of a radar apparatus is arrayed.
In this manner, a radio transmitter-receiver device is generally disposed behind the back surface of the front grill of a vehicle. However, there are many cases where the front grill is metal plated, so that it is difficult to transmit radio waves satisfactorily through the highly conductive metal. Also, front grills have a structure where air holes for taking in air are disposed, and front grills do not have uniform thicknesses. Therefore, when radio waves pass in and out through such a front grill, there is a difference in a radio transmission speed between the thin portion and the thick portion of the front grill, so that it is difficult to obtain good radar sensitivity.
On account of these reasons, in general, a window portion that is capable of transmitting radio waves is disposed in a portion of the front grill that corresponds to the portion where the radio transmitter-receiver device is disposed. When the window portion is disposed in the front grill, radio waves can pass in and out through the window portion. However, by including a window portion, the appearance of the front grill loses continuity and the appearance of the vehicle may be spoiled, since the internal portions of the vehicle, such as the radio transmitter-receiver device and the engine area, are viewed.
Therefore, conventionally, unity is provided between the window portion and the front grill body by inserting a radio transmission cover as disclosed in JP Patent Publication (Kokai) No. 2000-159039 A into the window portion of the front grill, for example. The radio transmission cover as disclosed in JP Patent Publication (Kokai) No. 2000-159039 A is formed by laminating a plurality of resin layers formed with concavity and convexity. This covering component can provide an impression by a metal layer deposited with concavity and convexity between the resin layers such that the fin member of the front grill continuously exists in the radio transmission cover.
Indium is used as a metal deposited in such a radio transmission cover. When depositing indium on a deposit member, indium is not deposited on the surface of the deposit member in a uniform film manner, but deposited in a minute insular manner. In other words, when indium is deposited on the deposit member, the surface of the deposit member has a minute mixture of a deposit portion where indium is deposited in a minute insular manner and a non-deposit portion where nothing is deposited. In this case, radio waves can pass in and out through the non-deposit portion and the surface of the deposit member can be recognized upon viewing as a member that has metallic luster, since the deposit portion has indium deposited in a minute insular manner.
However, as indium is an expensive metallic material, the increased material cost when the deposition is performed using indium is problematic. Moreover, it is difficult to form the deposited portion and the non-deposited portion in a well-balanced manner. For example, there is a case where radio waves do not pass in and out satisfactorily when the deposit portions are formed extremely closely together.
JP Patent Publication (Kokai) No. 2003-252137A discloses a method for manufacturing a radio transmission cover in order to enable processing of a metallic film using various metallic materials in addition to indium, and to provide a method for manufacturing a radio transmission cover in which radio waves pass in and out satisfactorily. The method comprises a first step for forming a cover base member in a planar table manner, a second step for forming the metallic film on one surface of the cover base member such that the shape of the metallic film corresponds to the shape of the aforementioned fin portion, a third step for forming a bright portion in an insular manner by physically or chemically removing a part of the metallic film, and a fourth step for forming a transparent resin layer on the upper layer of a plane where the bright portion of the cover base member is formed. The radio transmission cover is disposed on the front grill of a vehicle, the front grill having a reticular fin portion, and a radio transmitter-receiver device is disposed on the back surface side thereof.
JP Patent Publication (Kokai) No. 2000-159039 A and JP Patent Publication (Kokai) No. 2000-49522 A disclose a thin metallic layer comprising indium deposited on a metal portion area that can be viewed from the outside at a plastic-plated member for the beam path of a radar apparatus. However, it is necessary to ensure a bright design and the reliability of durability for radio transmittance by forming a stable protective layer so as not to allow the indium bright film layer to undergo exfoliation or be damaged by an external force, or to be corroded by an external environmental stress such as water or polluted air.
This is due to the following facts: indium is a very soft metallic material with a value of 1.2 in the Mohs hardness scale; indium corrodes under the aforementioned environmental stress since it is basically a metallic material; it is necessary to ensure the reliability of durability by securing the film thickness with certainty such that the bright-effect design of indium can be obtained without thickening the indium film layer more than is necessary since a radio transmission loss occurs as a conductivity loss based on the fact that indium is basically a metallic material; and the indium layer melts due to the heat of molten resin when successively conducting secondary formation of the lining resin on a resin-molded component in which a film is formed on the surface of a base body in advance, since the melting point of indium is 156° C., which is extremely low, for example.
JP Patent Publication (Kokai) No. 2000-344032 A discloses a ceramic film comprising silicon dioxide disposed as a protective layer for preventing exfoliation and ensuring anticorrosion of an indium film.
Meanwhile, an armoring component of a radar apparatus that has metallic luster is required to be resistant to attenuation of radio wave intensity so as to prevent the radar apparatus from receiving radio waves erroneously. In other words, it is necessary to minimize refraction and reflection of radio waves among members and at a metallic film in order to prevent the attenuation of radio waves in the armoring component per se.
JP Patent Publication (Kokai) No. 2002-135030 A discloses an armoring component for vehicles in terms of order to provide a radio-transmitting armoring component that is good in radio transmittance while having a design featuring a metallic luster. The armoring component for vehicles comprises an outer covering board composed of a radio-transmitting and transparent material, a metallic film formed in the inside portion of the outer covering board, and a back covering board composed of a radio-transmitting material. In the armoring component for vehicles, the relative dielectric constant of materials that constitute the outer covering board and the back covering board are substantially the same. In general, refraction and reflection of radio waves are likely to occur at the interface of different types of materials in proportion to the increase in the difference of the relative dielectric constants among the materials. The present invention thus can minimize the refraction and reflection of radio waves between the outer covering board and the back covering board in the radio-transmitting armoring component by using an outer covering board and a back covering board whose relative dielectric constants are substantially the same, thereby improving the radio transmission properties of the radio-transmitting armoring component. Specifically, examples of materials of the outer covering board include polycarbonate (PC) or polymethyl methacrylate (PMMA), and examples of materials of the back covering board include syndiotactic polystyrene, polyphenylene ether (PPE), polyvinyl chloride (PVC), and acrylonitrile/butadiene/styrene copolymer (ABS).
However, the board thickness of polycarbonate disclosed as the materials of outer covering board of a metallic layer in the aforementioned Patent Document 5 requires a thick design in light the need to reduce the amount of transmission attenuation of millimeter radio waves. The thickness of a transparent polycarbonate resin is about 3.5 mm when the value of the radio transmission attenuation is minimized in the 76 GHz band of millimeter waves. Even a board with a thickness of 3.5 mm or less can maintain its function if the radio transmission properties are not taken into account. However, in the design for disposing the molded component, it is an object to reduce the board thickness in order to increase the degrees of freedom for the design and also to reduce the mass. In other words, the performance of the component can be improved and the degrees of freedom for the design can be increased by employing materials for the resin layer with smaller transmission attenuations regarding millimeter radio waves.